Blanking dies are used to cut openings in sheet stock, the workpiece, in the manufacture of various products. Typically, these blanking dies consist of a punch and mating die which are arranged to engage the workpiece to cut the opening and then to disengage so that the workpiece can be advanced to the next station for additional operations. As the punch approaches the die it engages the surface of the workpiece, pushing it against the die opening and forcing a slug of the material through the opening and into an area where the scrap slugs are collected and removed from the tooling. Occasionally, one of these scrap slugs sticks to the end of the punch, after blanking, and is pulled out of the die opening. This is known as "slug pulling" in the industry. This loose slug then may catch on the edge of the freshly cut opening in the workpiece and result in a misfeed of the workpiece or it may remain on the end of the punch and interfere with the next blanking operation. This can cause serious damage to the tooling or the press. In any case, this results in a damaged product that must be identified and discarded. This can be extremely difficult to do when high speed stamping multi-out, progressive tooling modules that are producing thousands of products per minute are involved. It is very desirable to prevent the slug pulling in the first place. In conventional punch press applications where a reciprocating punch is arranged vertically above a fixed die, a vacuum system is most frequently used to augment the effect of gravity to pull the slugs downwardly away from the punch and into an exit cavity within the die tooling. However, this is not always an effective solution. In a horizontally arranged punch and die tooling module, however, the effects of gravity are not helpful and, if the stock being blanked is thin, the slugs will have very little mass and tend to adhere to the end of the punch more readily, especially if a thin film of lubricating oil is present on the surface of the stock. Of the attempts to control the removal of scrap and prevent slug pulling in this case, the most prominent include either a spring loaded member that is urged against the stack of slugs, as disclosed in U.S. Pat. No. 4,974,479 which issued Dec. 4, 1990 to Bakermans et al., or a protrusion in the slug exit opening that interferes with the slug thereby holding it within the opening. The later structure is disclosed in U.S. Pat. No. 5,136,907 which issued Aug. 11, 1992 to Bakermans et al. and which is incorporated herein by reference. The '907 patent discloses a punch and die arrangement wherein a camming rib is provided along one of the interior walls of the die opening that extends into the opening a small amount. There must be a small amount of additional clearance, than would otherwise be necessary, between the punch and the walls of the die opening so that as the punch enters the die opening it does not interfere with the camming rib. The bottom surface of the slug, however, is the same size as the die opening and, therefore, will interfere with the camming rib which tends to hold the slug in the die opening when the punch is withdrawn, although, slug pulling is not positively prevented.
What is needed is a mechanism that positively prevents slug pulling while maintaining normal clearances between the punch and mating die opening and, without the need for complex spring loaded members for holding the slug within the die opening.